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Chapter: Biochemistry: Carbohydrate Metabolism

Diabetes Mellitus

Diabetes mellitus is an important disorder of carbohydrate metabolism.

Diabetes Mellitus

Diabetes mellitus is an important disorder of carbohydrate metabolism. However, fat and protein metabolism are also affected in diabetic condition. Diabetes means excretion of excessive volume of urine and mellitus means sweet. So the word diabetes milletus refers to chronic excretion of large volume of urine containing glucose.

Diabetes mellitus, caused by a deficiency in the secretion or action of insulin, is a relatively common disease. Insulin is an endocrine hormone which is secreted by b-cells of islets of Langerhans of pancreas. The abnormality in glucose metabolism is indicative of diabetes or a tendency towards the condition. Diabetes mellitus is really a group of diseases in which the regulatory activity of insulin is defective.

There are two major clinical classes of the disease :

·              Type-I or insulin dependent diabetes mellitus (IDDM)

The disease begins early in the life and quickly becomes severe.

·              Type - II or non-insulin dependent diabetes mellitus (NIDDM)

The disease is slow to develop, milder and often goes unrecognized.

Type one requires insulin therapy and careful, life long control of the balance between glucose intake and insulin dose. The decreased or defective production of insulin is characterised by the following symptoms.

i.                  Decreased permeability of the cell membrane for glucose resulting in the accumulation of glucose in the blood. This condition is known as hyperglycemia. Glucose concentration increases as high as 500 mg/100 ml of blood.

ii.                  Polyuria: This means excretion of increased quantity of urine. This is to excrete the additional quantity of glucose in urine (glucosuria).

iii.                  Polydipsia: The excessive thirst which leads to increased consumption of water. This condition is known as polydipsia. This is to replace the volume of water excreted due to polyuria.

iv.                  Polyphagia: Excessive appetite leads to polyphagia and increased intake of food. This is to replace the lost nourishment. The diabetic has voracious appetite, but inspite of over eating, they lose weight and become lean and emaciated.

v.                  As glucose is not enough for energy production, increased mobilisation of fat from adipose tissue occurs. But the metabolism of fat is incomplete resulting in the production of large amounts of the intermediary products of fat metabolism namely ketone bodies (eg. Acetoacetate and b-hydroxybutarate). This condition is known as 'ketosis' and excess ketone bodies cause severe acidosis, ultimately resulting in 'coma'.

vi.                  Deposition of lipids in the walls of the blood vessels resulting "atherosclerosis".

Biochemical measurements on the blood and urine are essential in the diagnosis and treatment of diabetes, which causes profound changes in metabolism. A sensitive diagnostic criterion is provided by the glucose tolerance test (GTT).


Glucose Tolerance Test (GTT)


After a night without food, the patient drinks a test dose of 100 g of glucose dissolved in a glass of water. The blood glucose concentration is measured before the test dose and at 30 min intervals for several hours thereafter. A normal individual assimilates the glucose readily, the blood glucose rising to no more than about 80 to 120 mg/100 ml; little or no glucose appears in the urine. Diabetic individuals assimilate the test dose of glucose poorly; their blood glucose level far exceeds the kidney threshold (about 180 mg/100ml), causing glucose to appear in their urine.


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